Off-gases from smelt-reduction furnaces are of high temperature and may contain significant quantities of reducing gases such as carbon monoxide and hydrogen. It would clearly be of economic advantage to recover at least some of the sensible heat and make use of some of the reducing potential of the gases.
A number of prior art processes are known which provide either pre-heating or pre-reduction of metal oxide ores. Two processes, one by the Kawasaki Steel company and the other by the Nippon Kokan company, combine both features.
Generally speaking, the known processes entail one or other of the following disadvantages, and sometimes more than one:
several treatment steps may be required; PA1 expensive coke or other reductants may be required; or PA1 there may be temperature limitations leading to low pre-reduction, high residence time or low pre-heat temperature achieved.
If the temperature to which the oxide ore is exposed is too high, the particles may soften, leading to accretion on the apparatus and/or agglomeration of particles.
In the case of chromite ore, the pre-heating and/or pre-reduction processes described in the prior art are generally speaking limited to temperatures of around 1200.degree. C., at which temperature reduction is very slow.
The specification of U.S. Pat. No. 4,566,904 discloses a process for using exhaust gases from a melting crucible to pre-reduce iron ore. The exhaust gases are first reduced and cooled with a reductant such as natural gas. The cooled reducing gas is then used to pre-reduce the iron ore in a shaft furnace, a circulating fluidized layer or a fluidized bed. The optimum temperature of the cooled reducing gas if 900.degree. C. which is considerably less than the temperature of the exhaust gas. Consequently this process involves a considerable loss of sensible heat which is used to increase the concentration of reductants in the exhaust gas.
The specification of U.S. Pat. No. 4,629,506 describes the production of ferrochromium from a ferriforous chrome ore. The ore is heated in a rotary kiln for 20 minutes to 2 hours to a temperature in the range from 1480.degree. C. to 1580.degree. C. An atmosphere containing carbon monoxide is maintained inside the kiln to reduce the ore. The resultant plastic mass is cooled, crushed and magnetically separated into a coal rich fraction and a metal rich fraction. Preferably, the metal rich fraction is separated by dry density separation into a metal poor slag fraction and a metal rich alloy fraction, the slag rich fraction is crushed and a metal rich slag fraction is extracted by magnetic separation. The metal rich slag fraction is added to the metal rich fraction and both are then melted in a crucible for further processing.
The process described in U.S. Pat. No. 4,629,506 involves a number of process steps thereby resulting in additional capital and operating expenditure when compared with a process requiring less steps. Furthermore, the exhaust gases from the crucible are preferably used as carrier gases for blowing coal and ore into the crucible or for low temperature coking of the coal.
U.S. Pat. No. 4,851,040 describes a process for producing iron from fine grained iron ore by direct reduction. The process involves inserting sponge iron and coal fines or a low temperature carbonized coal into an iron bath and injecting oxygen to produce a reducing gas and iron. The reducing gas is used to reduce a pre-heated carbon coated fine grained iron ore in a "fluidized bed" at a temperature in the range from 700.degree. C. to 1100.degree. C. Spent reducing gas is used to preheat the fine grained ore to a temperature in the range from 450.degree. C. to 700.degree. C. as well as coat the grains of ore with fine grains of carbon. The fine grains of carbon are deposited on the grains of ore by decomposition of carbon monoxide. The carbon layer on the grains of ore prevents the grains from sticking during the reduction phase.
A process developed by Kawasaki Steel KK uses fine, unagglomerated ore; see JP 59080706. Pre-heating and pre-reduction is performed in a fluidized bed. Heat and carbon monoxide-rich reducing gas are supplied from the smelting reduction furnace off-gas and are supplemented by the injection of hydrocarbon gas. The pre-mixing of the furnace gas at 1350.degree.-1400.degree. C. with cooler hydrocarbon gas, for example, methane or propane, results in a cooler gas mixture such that the bed temperature is about 1200.degree. C. It is believed that, at this temperature, a residence time of 12-15 hours is required for substantial reduction of South African chromite with a mean particle diameter of 325 .mu.m. It is also believed that only limited reduction of the iron and chromium oxides was achieved when furnace gas, comprising carbon monoxide, was used alone. The hydrocarbon gas makes major contribution to the reduction of the chromite.
It is believed that the disadvantages of this process are the need for addition of hydrocarbon gas to achieve substantial reduction, and the low temperature of the fluidised bed, which causes the low rates of reaction but is necessary to prevent softening of the chromite feed and subsequent agglomeration of the particles within the fluidised bed. It seems that the consequent high residence time prevents all chromite ore feed from being pre-heated and pre-reduced. (Some chromite ore is injected directly to the smelting reduction furnace.
An object of the present invention is the provision of a process for partly or almost completely reducing fine to coarse-grained metallic oxides, in particular metal ores, whereby the metallic oxide particles do not agglomerate in any appreciable amount but exist after the prereduction in the form of a granular, pneumatically conveyable material, and these partly reduced particles can be supplied to a final reducing process, preferably a smelting reduction process, without requiring any further elaborate processing steps.
Accordingly the present invention provides a process for pre-heating and pre-reducing a metal oxide which process comprises forming a stream of metal oxide particles and hot reducing gas to heat and to reduce at least partially the metal oxide particles wherein the hot reducing gas has a temperature in excess of that at which the metal oxide particles, particles contained in the stream of reducing gas or both exhibit sticky characteristics.
Metal oxide particles exhibit sticky characteristics when heated to a temperature range in which one or more phases present in the oxide exist as liquids and the remaining phases continue to exist in the solid state. This normally occurs first at a eutectic or peritectic point in the multi-component system. When all the phases present exist in the liquid form, the metal oxide particles no longer exhibit sticky characteristics. The temperature at which metal oxide particles begin to exhibit sticky characteristics and the range of temperatures over which they exhibit sticky characteristics varies from one metal oxide to another and one mineral mixture to another. On the low side are systems such as Al.sub.2 O.sub.3 -FeO-SiO.sub.2 and FeO-Fe.sub.2 O.sub.3 -SiO.sub.2 where certain compositions are liquid at 1150.degree. C. On the high side are systems such as CaO-MgO-SiO.sub.2 in cases where the bulk of iron or other reducibles have been extracted from fluxes and gangue minerals. Such systems have melting points around 1300.degree. to 1350.degree. C. In general stickiness associated with metallization of iron can occur at temperatures down to 600.degree. C. but is most rapid at temperatures in excess of 1000.degree. C. This threshold is increased by 100.degree.-200.degree. C. when heating ores containing manganese or chromium.
The hot reducing gas may be a synthesis gas derived directly from the combustion of natural gas or coal in the presence of steam. Preferably, however, the hot reducing gas is an off-gas from a smelting reduction furnace. Such off-gases normally contain carbon monoxide and some hydrogen. In addition hot off-gas from a smelting reduction furnace frequently contains particles of metal oxide ore, particles of ore that have been partially reduced and droplets of metal. The outlet temperature of such off-gases normally exceeds the sticky temperature. Consequently, when off-gases are cooled, care must be taken to avoid agglomeration of the particles and accretion thereof on apparatus.
Agglomeration and accretion of sticky particles can be avoided by employing either or a combination of two techniques. The first technique involves rapidly heating the metal oxide particles to a temperature well in excess of the sticky temperature range and after a short interval rapidly cooling the particles to a temperature below the sticky temperature range. The second technique involves heating the metal oxide particles to a somewhat lower temperature which nevertheless involves heating some of the particles to a temperature in excess of the sticky temperature range and causing the particles to enter flow patterns such that inter-particle collisions of hot sticky particles and collisions of hot sticky particles with apparatus are minimized. This ensures that the metallic oxide particles can be maintained for a longer period within the sticky temperature range. This can be accomplished by first reducing the velocity of the stream as it flows from a first end of a vertical chamber towards a second end and subsequently increasing the velocity as the stream approaches the second end. In this way incoming particles are initially entrained or if present in the hot reducing gas remain entrained in the higher velocity entrance stream but as the stream slows as it moves towards the second end some of the particles diverge from the stream and fall back towards the first end. Whether or not a particle will remain entrained depends upon a number of factors including its density, size, surface area, surface roughness and its position in the stream. Particles remaining entrained in the stream cool below the sticky temperature range before reaching the outlet in the second end and particles which fall towards the first end also cool below the sticky temperature range before becoming entrained again in the higher velocity entrance stream of reducing gas.
This invention also provides a treatment chamber for pre-heating and pre-reducing metal oxides by the process according to the invention. The internal configuration of the chamber and the inlet ducts for reducing gas are chosen to promote pre-heating and pre-reduction while minimising agglomeration and accretion.
The treatment chamber comprises a body portion, a first end, a second end, an inlet in the first end and an outlet in the second end. The body and each end are defined by walls which taper from the body portion towards the inlet and the outlet respectively. The body portion has a cross-sectional area that is many times larger than that of the inlet and the outlet. Furthermore, the chamber is sufficiently elongated to ensure that the flow pattern hereinbefore described can be set up when the chamber is vertically oriented.
This invention further provides an apparatus for the smelting of a metal oxide ore, which incorporates the treatment chamber defined above.
A surprising advantage of the present invention is that it permits off-gases to be quenched while ensuring that any sticky solids or other entrained material are cooled to temperatures at which agglomeration and accretion is much reduced or prevented altogether. However, if the off-gases contain a relatively high proportion of sticky solids it may be necessary to alter certain variables of the process such as the rate of injection of fresh solid particles.